The present invention relates to a technique for detecting back-scattering light from a scattering potential which has a scattering center at a micro object or the like located within, for example, a living body, which is a medium that strongly scatters light; obtaining information regarding a scattering position and information regarding reflection amplitude by use of interference measurement means which utilizes a phenomenon that coherence is present even in reflection light from an object that strongly scatters light and which utilizes the shortness of coherence length of low-coherence light; and obtaining single-dimensional or two-dimensional image data, or multidimensional image data such as three-dimensional image data, while scanning the interior of the object. More particularly, the present invention relates to an optical interference tomographic image observing apparatus which enables easy observation of a tomographic image of a light-scattering medium such as a living body by use of a remote device.
An attempt for obtaining a reflection tomographic image of a living body, which is a medium that strongly scatters light, starts from construction of an interferometer by use of low-coherence light (see Naohiro Tanno, xe2x80x9cKogakuxe2x80x9d Vol. 28, No. 3, pp. 116-125 (1999)). A conventional technique will be described with reference to FIGS. 1 and 2.
FIG. 1 is a diagram showing the structure of a conventional light-wave reflection image measurement apparatus proposed by the present inventors.
In this light-wave reflection image measurement apparatus, a light beam from a low-coherence (also referred to as partial-coherence) light source 71 is introduced directly to a Michelson""s interferometer in order to split the beam into two beams by means of a beam splitter 73. One of the split beams, which is to be used as reference light, undergoes frequency shift. The frequency-shifted light beam is reflected by a movable reflection mirror 72, which also serves to change a depth position within an object, and is caused to enter a photo detector 75. The other light beam or transmission light is supplied to an object 74 to be measured as object irradiation light. The light is scatter-reflected by a layer of scattering objects located at a deep portion of the object 74 and having a different refraction index. The reflection light, serving as object reflection light, is mixed with the reference light by means of the beam splitter 73 so as to cause interference. As a result, a beat signal is detected by means of the photo detector 75. While the positional relation between the illumination light and the object is changed in order to effect scanning, the detected electric signals are fed to a computer via a filter and an amplification/signal processing section, whereby the detected electric signals are stored in the computer. The thus-stored electric signals are converted to image data in order to obtain a reflection tomographic image.
FIG. 2 is a diagram showing the structure of a conventional tomographic-image observation apparatus which employs a structure on the basis of the above-described principle and in which optical fibers are disposed to form optical paths in order to cope with vibration and facilitate handling (see, for example, Japanese Kohyo (PCT) Patent Publication No. 6-511312).
As shown in FIG. 2, a light beam from a light source 81 propagates through a fiber 82 and enters a splitter/mixer circuit 86. One light beam emitted from the splitter/mixer circuit 86 propagates through a fiber. Light coming out from the outgoing end of the fiber is converged by means of a convex lens 83. As a result, the light is reflected by an object 84, and the reflection light serves as object reflection light. After impartment of a frequency shift by means of a piezo-oscillation phase shifter 85, the other light beam emitted from the splitter/mixer circuit 86 is reflected by a movable reflection mirror 80, and the reflection light serves as reference light, which is mixed and is caused to interfere with the object reflection light by means of the splitter/mixer circuit 86. The mixed light enters a photo detector 87, whereby a reflection tomographic image can be observed in the same manner as described above.
Conventional interference measurement methods all utilize a movable reflection mirror for changing light reflection position (reference reflection position). In general, the movable reflection mirror is a reflection mirror attached to a linear actuator or a galvano-motor. Since the linear actuator moves an object back and forth via gears, the moving speed is as low as several mm/sec. In another method, a long fiber is wound around an electrostrictive element such as an element made of PZT, and the length of a reflection light path is changed through extension and contraction of the fiber.
Among the above-described conventional methods, the method employing a reflection mirror attached to a linear actuator or the like involves problems in that high-speed sweeping is difficult, and that when the mirror is moved back and forth periodically, linearity is deteriorated due to backlash and other causes.
Meanwhile, the method in which a long fiber is wound around an electrostrictive element such as an element made of PZT and the length of a reflection light path is changed through extension and contraction of the fiber involves a problem in that since a path for reference light becomes excessively long, the temperature varies, and the length of a path for object light must be increased.
Furthermore, the apparatus utilizing a linear actuator and the apparatus utilizing an electrostrictive element are both large in size, and fabrication of a compact, transportable apparatus including an interferometer is difficult.
Moreover, when sweeping speed is low, a very long time is needed to complete tomographic image measurement, which makes applying the tomographic image measurement to examination of a living body or a moving object difficult. In addition, when the path for reference light and the path for reflection light are made excessively long, optical signals attenuate excessively. In this case, the SN ratio of an obtained image decreases, which makes observation of a deep portion of an object difficult.
In order to solve the above-described problems, the present invention proposes a method and a specific apparatus which utilizes a rotating Littrow reflector prism and which can reflect a light beam in order to produce a delay reflection light beam or a progressive reflection light beam which travels toward the incoming direction of the light beam, even when the reflection point moves along a circumference of a rotary body upon rotation thereof or a surface of the prism facing the vertex thereof inclines. The method and the apparatus utilize the features of the prism such that the prism reflects a light beam toward the direction from which the light beam comes, and even when the incoming light beam inclines with respect to the surface facing the 90-degree vertex, the prism accurately reflects a light beam toward the incoming direction. Further, the present invention realizes a reliable, high-speed-scanning reflection mirror by attaching prisms on a small, high-speed motor, and opens to the road to a compact, simplified, transportable apparatus which can be used practically for optical interference tomographic image observation, which is an object of the present invention. Moreover, another object of the present invention is to provide an optical interference tomographic image observing apparatus which extracts reflection signals of wide dynamic range and high SN ratio through high-speed scanning in order to detect a static or dynamic structure of a deep portion of, for example, a living body and to produce a multidimensional image for observation.
In order to achieve the above objects. the present invention provides the following.
[1] An optical interference tomographic image observing apparatus, characterized by comprising a rotary prism apparatus which includes a Littrow reflector prism having a 90-degree vertex and disposed near a circumference of a rotary body in such a manner that a surface facing the vertex extends substantially perpendicular to a tangential line of the circumference, the prism having a characteristics such that when a light beam impinges the surface, the light beam is reflected in a direction parallel to the incidence direction, wherein through utilization of the characteristics, the reflection point can be scanned in a predetermined direction as the rotary body rotates; and a delay reflection light beam is periodically generated when the rotary body rotates in the travel direction of the light beam and a progressive reflection light beam is periodically generated when the rotary body rotates in the opposite direction.
[2] An optical interference tomographic image observing apparatus as described in [1] above, further comprising:
means for splitting a light beam from a low-coherence light source into two light beams, one of the light beams, serving as reference light, being delayed or advanced by means of rotary scanning of the reflection point in order to obtain a reflection light beam having a Doppler shift frequency, and the other light beam being converged to an object to be measured which has a multilayer structure in terms of refraction index distribution; an objective lens for capturing object reflection light from a scattering potential portion at a deep portion of the multilayer object; a photo detector for performing heterodyne detection for obtaining a beat signal of the shift frequency, which is generated on the basis of the low coherence, characterized in that a maximum interference signal can be obtained only when the reference light and the object reflection light merge together after passage through respective optical paths having the same optical path length as measured from the split point; means for calculating, in the form of coordinates, the scanned reflection point of the delay or progressive reflection light beam; and a signal control processing system, a computer, and a display which measure and display a reflection tomographic image, while using, as image data, the coordinates and an amplitude of the beat signal representing reflection light from the scattering potential at the deep portion of the object to be measured.
[3] An optical interference tomographic image observing apparatus as described in [2] above, wherein the means for calculating, in the form of coordinates, the scanned reflection point of the reflection light beam includes a photo detector for capturing deflection angle reflection light from the rotary prism, wherein the photo detector generates a timing pulse upon detection of the deflection angle reflection light before generation of the reflection light beam; and the scanned reflection point is calculated from the rotation frequency, rotation circumferential length, and rotation angle of the rotary prism, and is used as a coordinate of the scattering potential.
[4] An optical interference tomographic image observing apparatus as described in [2] above, wherein the travel direction of the light beam emitted from the low-coherence light source is referred to as a Z axis; a semi-transparent reflection mirror is provided as the means for splitting the light beam into two beams; the objective lens is disposed in a direction toward which a light beam passing through the semi-transparent reflection mirror travels, the light beam serving as object irradiation light; a direction along which a reflection light beam from the semi-transparent reflection mirror serving as reference light travels is referred to as a Y axis; the light source, the semi-transparent reflection mirror, and the objective lens are integrated into a unit structure; and a mechanism for rotating the unit structure about the Y axis is provided in order to rotate the unit structure to thereby sweep the irradiation point on the object to be measured along the X-axis direction, whereby observation of a two-dimensional tomographic image on an X-Z plane is enabled.
[5] An optical interference tomographic image observing apparatus as described in [2] above, wherein the respective means are accommodated within a casing; a dielectric multilayer film reflection mirror which reflects only the wavelength band of the low-coherence light source is disposed before the photo detector in order to reflect and guide the mixed-wave interference wave to the photo detector; a light source whose wavelength band differs from that of the low-coherence light source is provided; a second half mirror is provided in order to reflect light emitted from the second light source and cause the light to pass through the dielectric multilayer film reflection mirror, the half mirror, and the objective lens in order to radiate the object to be measured, reflection light from the surface of the object traveling back along the above-described optical path, and passing through the second half mirror; a CCD camera is provided in the same casing in order to capture the image of the surface having magnified by the objective lens; and a display is disposed outside the casing in order to enable previous observation of a measurement position on the object.
[6] An optical interference tomographic image observing apparatus as described in [5] above, wherein the casing is equipped with a grip handle which has a switch for starting acquisition of measurement data of the tomographic image after positioning of the measurement point through observation of the measurement point.
[7] An optical interference tomographic image observing apparatus as described in [4] above, further comprising a rotation mechanism which rotates about the X axis and which receives the casing on which the unit structure is disposed at an angle of 90 degrees, whereby, in addition to the observation of a two-dimensional tomographic image on an X-Z plane, scanning along the Y-axis direction is effected by the rotation mechanism in order to enable observation of a three-dimensional tomographic image.
[8] An optical interference tomographic image observing apparatus as described in [7] above, wherein the objective lens is replaced with an objective lens for funduscopy; and the object irradiation light is scanned by use of a galvano-mirror.
[9] An optical interference tomographic image observing apparatus as described in any one of [1] to [7] above, wherein the optical path for reference light is turned up and down by a group of reflection mirrors in order to increase the length of the optical path; and an optical fiber having a length corresponding to the increased length is disposed in the optical path extending between the half mirror for splitting and the object, whereby remote measurement is enabled.
[10] An optical interference tomographic image observing apparatus as described in any one of [1] to [7] above, wherein an optical fiber is disposed in the optical path for reference light in order to increase the length of the optical path; and an optical fiber capable of transmitting images and having a length corresponding to the increased length is disposed in the optical path extending between the half mirror for splitting and the object, whereby remote measurement is enabled.